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Polymer Science
Introduction to Fibre Science and Rubber Technology
B. Rubber Technology
Natural and Synthetic Rubber
Dr. Utpal Kumar Niyogi Deputy Director
Division of material Science
Shri Ram Institute for Industrial Research
19, University Road
Delhi 110007
(23.07.2007)
CONTENTS
Natural Rubber
Latex technology
Latex compounding
Dry rubber technology
Properties of raw natural rubber
Synthetic Rubber
Styrene butadiene rubber Polybutadiene
Nitrile rubber
Neoprene rubber
Ethylene propylene rubber Butyl rubber
Chlorobutyl rubber
Polysulfide rubber
Silicone rubber
Fluorocarbon elastomers
Thermoplastic elastomers
Key Words
Tapping, Coagulation, Masticate, Compounding, Scorch, Shear,
Gel, Coagulum, Crystallization, Abrasion, Flex,
Gum, Fatigue, Ageing, Branching, Impregnation, Damping,
Encapsulation, Potting, Polydispersity
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Natural Rubber
Introduction
The natural rubber (NR) presently used by industry is obtained
by tapping the sap known as
Latex, from the large forest tree Hevea Brasiliensis, which
occurs in the southern equatorial region of America. By the end of
eighteenth century the properties of rubber as obtained
from the Hevea tree available at that time entirely in the
forest of Amazon valley, were
known throughout Europe. The Europeans found that by
systematically tapping the tree, the
latex can be extracted regularly. With the development of
plantation in the Far East, it was
found that latex could be preserved by adding ammonia to it
immediately after it is collected.
This marked the beginning of our commercial latex technology.
Presently apart from Brazil,
vast plantations are in existence in India, Malaysia, Indonesia,
Sri Lanka, Vietnam, Cambodia and Liberia.
Tapping is usually done by shaving about one or two millimeters
thickness of bark with each
cut, usually in the early morning hours, after which latex flows
for several hours and gets
collected in cups mounted on each tree. The cut is made with
special knife or gouge, sloping
from left to right at about 20-30 from the horizontal. The
content of each latex cup is
transferred to five-gallon containers and transported to storage
tanks at bulking station.
The latex may either be concentrated to about 60% dry rubber
content (DRC), usually by
centrifuging or evaporation, or alternatively coagulated or
dried. The two approaches lead to
two distinct branches of rubber technology, namely latex
technology and dry rubber
technology.
Latex Technology
Latex technology is a highly specialized field that is not too
familiar to most polymer
chemists and even many rubber compounders. The art and science
of handling latex
problems is more intricate than regular rubber compounding and
requires a good background
in colloidal systems. While latex differs in physical form from
dry rubber, the properties of
the latex polymer differ only slightly from its dry rubber
counterpart. Unlike the dry rubber,
which must be masticated (mechanically sheared) before use, the
latex polymer need not be
broken down for application, thus retaining its original high
molecular weight which results
in higher modulus products. Other advantages enjoyed by
applications involving latex are,
lower machinery costs and lower power consumption, since the
latex does not have to be
further processed into dry form and compounding materials may be
simply stirred into the
latex using conventional liquid mixing equipment.
Composition of Rubber Latex
The natural product, which is exuded as a milky liquid by the
Hevea tree, is a colloidal
solution of rubber particles in water; the particle diameters
range between 0.05 and 5 . It
is a cytoplasmic system containing rubber and non-rubber
particles dispersed in aquous
serum phase.
Freshly tapped Hevea latex has a pH of 6.5 to 7.1 and density
0.98 g/cm3. The total solids of
fresh field latex vary typically from 30 to 40 wt % depending on
clone, weather, stimulation,
age of the tree, method of tapping, tapping frequency and other
factors. The dry rubber
content is primarily cis-1,4,- poly isoprene,
CH3
CH2 C CH CH2
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The non-rubber portion is made up of various substances such as
sugars, proteins, lipids,
amino acids and soluble salts of calcium, magnesium, potassium
and copper. The solid phase
typically contains 96% rubber hydrocarbon, 1 wt % protein and 3
wt % lipids with traces of
metal salts.
Stabilization of Rubber Latex
Though fresh rubber latex is nearly neutral and the rubber
particles are stabilized by an
adsorbed layer of protein and phospholipids, but on exposure to
air the latex rapidly develops
acidity and within 12 to 24 hours spontaneous coagulation sets
in (at an approximate pH of
5). The latex has therefore, to be preserved immediately after
collection against rise in
acidity by bacterial putrefaction. As already mentioned, ammonia
has long been used as
preservative of latex owing to certain advantages including the
ease of its removal by
blowing air or reaction with formaldehyde. Other preservatives
such as sodium
pentachlorophenate, sodium salt of ethylene diamine tetraacetic
acid, boric acid or zinc alkyl
dithiocarbamates, may be used with smaller amount of ammonia.
This is known as low
ammonia latex and has the advantages of lower cost and
elimination of the need to
deammoniate the latex before processing into products.
Concentration of Rubber Latex
The ammonia preserved field latex which is known as normal
(un-concentrated) latex is not
suitable for commercial use as it contains considerable amount
of non-rubber constituents
which are detrimental to the quality of products and also
contains too much water which is
costly for transportation. The latex is, therefore, concentrated
to about 60% rubber solids
before leaving the plantation. This concentration process is
carried out either by centrifuging,
creaming, electrodecantation or evaporation.
The first two processes make use of increasing the gravitational
force of the rubber particles,
by applying centrifugal force on the former or by adding a
creaming agent like sodium
alginate, gum tragacanth etc. in the latter process. Both these
processes of concentration
result in a decrease of non-rubber content, the centrifuging
process being superior in this
respect.
The concentrated latex obtained by electrodecantation process
which utilizes the negative
charge on the tiny rubber particles, is similar in composition
to the centrifuged latex; however
cost economics does not favour this process to be exploited on
commercial scale.
The evaporated latex contains all the non-rubber constituents
present in the original normal
latex. It contains a small amount of ammonia. Because of its
high stability, evaporated latex
is useful in compounding heavily loaded mixes, hydraulic cement
etc.
The centrifuged latex is most widely used in industry. Latex
concentrate constitutes slightly
more than 8% of the global natural rubber supply, and about 90%
of this is centrifuge
concentrated. The term latex mentioned anywhere is this text now
onwards will mean the ammonia preserved centrifuged latex.
Principal outlets for natural rubber latex are foam
rubber, dipped goods and adhesives.
Latex Compounding
In latex technology, concentrated latex is first blended with
the various additives as required
for different applications. The blending of different additives
is known as latex
compounding. Latex compounding involves not only the addition of
the proper chemicals to
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obtain optimum physical properties in the finished product but
also the proper control of
colloidal properties which enable the latex to be transformed
from the liquid state into
finished product.
Viscosity control in the latex is very important. The particle
size of the latex has a great
effect on viscosity. Large particles generally result in low
viscosity. Dilution with water is
the most common way to reduce viscosity. Certain chemicals such
as trisodium phosphate,
sodium dinaphthyl methane disulfonate are effective viscosity
reducers.
Thickening Agents
Thickening may be accomplished with either colloidal or solution
thickeners. Small particle
size materials such as colloidal silica will thicken latex when
added to it. Solutions of such
materials as alpha protein, starch, glue, gelatin, casein,
sodium polyacrylates and poly (vinyl
methyl ether) will also thicken latex.
Wetting Agents
Sometimes the addition of a wetting agent to latex mix is
necessary for successful
impregnation of fabric or fibres with latex. Sulfonated oils
have been found to be effective in
assisting complete penetration between textile fibres without
any danger of destabilizing the
latex.
Vulcanizing Agents
Curing or vulcanization, which involves the chemical reaction of
the rubber with sulphur in
presence of an activator (such as zinc oxide) and accelerator,
manifests itself in an increase in
strength and elasticity of the rubber and an enhancement of its
resistance to ageing.
Vulcanization of latex may be effected by either of the two
ways; i) The rubber may be
vulcanized after it has been shaped and dried, or ii) The latex
may be completely vulcanized
in the fluid state so that it deposits elastic films of
vulcanized rubber on drying. The latter
process, however, does not yield products of high quality and is
resorted to only in the
production of cheaper articles, e.g. toy balloons.
The problem of scorching or premature vulcanization is rarely
encountered in practical latex
work and hence ultra accelerators such as zinc diethyl
dithiocarbamate (ZDC) alone or in
combination with zinc salts of mercaptobenzothiazole (ZMBT),
tetramethyl thiuram
disulphide (TMTD), polyamines and guanidines are used. The
latter two also function as gel
sensitizers, or secondary gelling agents, in the preparation of
foam rubber. The doses of the
vulcanizing ingredients are adjusted according to the
requirements of the end products. Thus
only small amount of sulphur and accelerator (0.5-1.0 phr) with
little or no zinc oxide are
required in the production of the transparent articles, whereas
in case of latex foams the doses
are quite high.
Antioxidants
Because of the great surface area exposure of most latex
products, protection against
oxidation is very important. Many applications involve light
colored products, which must
not darken with age or on exposure to light. Non-staining
antioxidants such as hindered
phenols (styrenated phenols) must be used. Where staining can be
tolerated, amine
derivatives such as phenylene diamines, phenyl
beta-napthylamine, ketone-amine
condensates may be used. These have good heat stability and are
also effective against
copper contamination, which cause rapid degradation of
rubber.
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Fillers
Fillers may be added to latex to reduce the cost of rubber
articles, to prevent spreading mixes
leaking through the fabric, to increase the viscosity of the
compound or to modify the
properties of the rubber. Most of the non black fillers such as
china clay, mica powder,
whiting (calcium carbonate), Lithopone, Blanc Fixe (barium
sulphate) may be used in latex
compounds. Carbon black does not reinforce latex in the manner
that it does dry rubber, and
is used only in small amounts in latex for color, as are various
other dyes and pigments.
Softeners
In applications like toy balloons, softeners are added to soften
them so that they may be
easily inflated. Softening agents in general used are liquid
paraffin, paraffin wax and stearic
acid.
Dispersing Agents
The particle size of solid materials added to latex must usually
be made as small as possible
to ensure intimate contact with the rubber particles. Solid
materials are usually added to latex
as dispersion. The material to be added is mixed with dispersing
agents in deionized water
and ground to a small particle size in a ball mill or attritor.
In these devices stones or other
hard pebble-sized materials are made to tumble and mix with
chemicals reducing them to
very small size.
The selection and amount of dispersing agent is determined by
the physical properties of the
material to be dispersed. The functions of these agents are to
wet the powder, to prevent or
reduce frothing and to obviate re-aggregation of the particles.
The concentration of
dispersing agents rarely exceeds 2% except in special
circumstances. None of the common
materials such as gelatin, casein, glue or soap such as ammonium
oleate possesses all the
requisite properties and hence it is necessary to use mixtures
of two or more of them. When
putrefiable dispersing agents such as casein, glue and gelatin
are used, a small amount of
bactericide, such as 0.01% sodium trichlorophenate may be
added.
Non putrifiable proprietory dispersing agents such as Dipersol F
conc. of Indian Explosives
Ltd. based on sodium salt of methylenebis [naphthalenesulfonic
acid] are also available
which are highly efficient dispersing agents with little foaming
tendency during milling.
Time, equipment and labour can often be saved by dispersing
together (in the correct
proportion) all the water insoluble ingredients required for a
particular compound including
sulphur, zinc oxide, accelerator, antioxidant, color and
fillers. Mixed dispersion having
excellent storage stability against reaggregation and settling
can be prepared by using the
following formula and method:
Mixed total solids - 100 parts
Dispersal F conc. - 4 parts
Deionized water - 96 parts
The mixed ingredients are dispersed by ball milling for at least
48 hours.
Emulsifying Agents
As in the case of dispersions, deionized water should also be
used for the preparation of
emulsion of water immiscible liquids to be used in latex
compounds. An emulsion is defined
as a system in which a liquid is colloidally dispersed in
another liquid. The emulsions use in
for latex should be the oil-in-water type in which water is the
continuous phase.
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Simple equipment for the preparation of emulsion consists of a
tank and a high-speed stirrer.
Very fine and stable emulsions can be prepared by using a
homogenizer. In a homogenizer,
the liquid mixed with the required amount of water and
emulsifying agent is forced through
fine orifice under high pressure (1000-5000 psi); the liquid mix
is thus subjected to a high
shearing force which breaks down the particles to the required
size.
Various synthetic emulsifying agents are available in the
market, but for use with latex, soaps
have been found to be quite satisfactory. For getting a
satisfactory emulsion, the soap is
produced in situ during mixing of the components. In this
method, the cationic part of the
soap (ammonia, KOH or amine) is dissolved in water and the
anionic part (oleic, stearic or
rosin acid) is dissolved in the liquid to be emulsified. Soap
forms when these solutions are
mixed. A method of preparation of a typical 50% emulsion of
liquid paraffin is given below:
Liquid paraffin - 50.0 parts
Oleic acid - 2.5 parts
Concentrated ammonia solution - 2.5 parts
Deionized water - 45.0 parts
The oleic acid is mixed with liquid paraffin and the mixture is
added to the water containing
concentrated ammonia solution. The two phases are mixed by
agitation and a stable emulsion
is obtained by passing through a homogenizer.
Stabilizers
The stabilizing system naturally occurring in ammonia preserved
latex is adequate to cope
with the conditions normally encountered during concentration,
transportation and
distribution but fails to withstand the more severe conditions
met with during compounding
and processing, when additional stability must be ensured by the
addition of more powerful
agents.
Some degree of stabilization may be attained by adding simple
materials such as soap and
proteins (e.g. casein). Casein is liable to putrefy and impart
to latex a high initial viscosity,
which may yield products having inferior physical properties.
Soaps are convenient to use
but their behaviour is not always predictable and they have
limited applications. Synthetic
stabilizers are now available which are free from the
limitations associated with soaps and
proteins.
An anionic surface-active agent such as sodium salt of cetyl /
oleyl sulphate when present in
sufficient quantity, stabilizes latex against heat, fillers and
mechanical working. It has no
thickening action on latex compounds, does not alter the rate of
cure and has no adverse
effect on the vulcanizate. It is most effective in alkaline
medium and loses its activity in
presence of acids and polyvalent ions. It is, therefore, most
suitable for the coagulant dipping
process. Its efficiency remains unaffected by the increase in
temperature.
A non ionic surface active agent such as an ethylene oxide
condensate possesses remarkable
stabilizing power to protect latex compounds against the effects
of mechanical action, acids,
polyvalent salts etc. It differs from anionic stabilizers in its
method of functioning. It
increases the hydration of the stabilizer film at the
rubber/water interface and has little or no
effect on the charge. Because of the high chemical stability,
its use is not recommended in
acid coagulant dipping process. However, it loses its activity
at elevated temperature and this
property is utilized in heat-sensitive compounds. It affords
excellent protection to such
compounds during storage at room temperature, but on heating it
loses this power and gelling
(or setting) of rubber particles takes place.
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Compounding Criteria
During compounding, it is essential to avoid the addition of any
material liable to cause
coagulation. As already discussed, the latex compound should be
properly stabilized. In
general, the addition of water-soluble organic liquids, salts of
polyvalent metals and acidic
materials are to be avoided. Water-insoluble liquids and solids
must be added as emulsions
and dispersions respectively, in which the size of the
individual particle is of the same order
as that of the rubber particles in the latex. Care should be
taken to avoid the use of hard
water at any stage of latex compounding as it has a
destabilizing action on latex.
The containers for the latex may be made from stone, enamelled
iron, stainless steel, and
wood lined with rubber or gutta-percha. It is preferably
thermostatically controlled against
changes in atmospheric temperature and is fitted with water
jacket. It is equipped with a
mechanical stirrer. During the addition of the compounding
ingredients, the mix should be
stirred slowly but thoroughly. Slow stirring of the latex mix
assists in the removal of bubbles
and minimizes the formation of a skin, which arises from
evaporation of water in the latex. It
is important to avoid contact between the stirrer and the
container, since latex is readily
coagulated by friction.
Processing of Latex Compound
After a suitable latex compound has been prepared, the next step
is to get the shape of the
article to be made, set the shape and then vulcanize. The
different latex processes classified
according to the method of shaping are: i) Dipping ii) Casting
and Moulding iii) Spreading
iv) Spraying v) Foaming
(i). Dipping: A variety of thin rubber articles e.g. toy
balloon, teats, gloves etc. can be
prepared from latex by dipping process. The process consists
essentially of dipping a former
in the shape of the article to be made into the compounded
latex. The formers may be made
from a variety of materials, including metal, glass, lacquered
wood and porcelain. The
deposited film is dried, vulcanized in circulating hot air,
steam or hot water and then stripped
from the former. This is known as straight dipping as against
coagulant dipping where the former is first coated by dipping into
a chemical coagulating agent. The coagulants may be
either salt coagulants or acid coagulants. A typical dipping
compound suitable for balloons,
gloves etc is given in Table 1.
(ii)Casting and Moulding: Casting involves the use of a mould on
the inside walls of which
the rubber article is formed, the pattern on the inside of the
mould determining the ultimate
shape of the article. The basic principle of latex casting is to
set the compound in the mould followed by subsequent drying,
removal from the mould and vulcanizing. Depending on the
technique of setting (gelling) inside the mould, two types of
moulds are used: i)Plaster of Paris moulds, and ii) Metal moulds.
Gelation in plaster mould is brought about by partial
absorption of water by the mould material and in a metal mould
by using a heat-sensitizing
agent.
Both solid and hollow articles can be produced by the process of
casting. In the preparation
of the solid articles the entire rubber latex content of the
mould is gelled and subsequently
dried. Non-porous metal moulds are used both for hollow and
solid articles whereas the
porous plaster moulds are generally used for hollow articles.
Hollow articles are produced by
forming the required thickness on the inside wall of the mould.
With a well-formulated
compound, satisfactory wall thickness can be built up in about
5-10 minutes. The plaster
mould, together with its deposited latex, is then placed in an
oven at 40-60C for several
hours. When the deposit is consolidated and partially dry, the
mould is removed from the
oven, allowed to cool and the article is carefully removed. It
is then washed, dried and cured
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for 30 minutes at 100C in air. A general formulation of latex
compound suitable for casting
in plaster of Paris moulds is given in Table 1.
Table 1: Typical formulation of latex compounds for different
applications
Ingredients Dipping
Compound
(Parts by
wt.)
Casting
Compound
(Part by wt.)
Carpet
Backing
Compound
(Parts by
wt.)
Spraying
Compound
(Parts by
wt.)
Foam
Compound
(Parts by
wt.)
60% Centrifuged
Latex
167.0 167.0 167.0 167.0 167.0
20% Non ionic
stabilizer Solution
1.0 - - - -
20% Anionic Surface
active agent
- 3.0 25.0 6.0 -
20% KOH Solution - - 1.5 1.0 -
20% Potassium oleate
soap solution
- - - - 5.0
50% ZDC dispersion 2.0 2.0 2.0 2.5 2.0
50% Sulphur
dispersion
2.0 3.0 3.0 5.0 4.0
40% Zinc oxide
dispersion
0.5 4.0 7.5 7.5 10.0
50% ZMBT dispersion - - - - 2.0
50% Phenolic
antioxidant Emulsion
0.5 2.0 2.0 2.0 2.0
20% Ketone-amine
Antioxidant dispersion
- - - 5.0 2.5
40% DPG dispersion - - - - 0.6
Sulphonated oil
wetting agent
- - - 0.75 -
50% Liquid Paraffin
Emulsion
3.0 - - - -
50% Filler (China
clay) dispersion
- 18.0 150.0 - 20.0
20% Pigment
dispersion
- 5.0 As required - -
20% Sodium
Silicofluoride
dispersion
- - - - 5.0
Fast Colour - - - - As required
Deionized Water (To
adjust viscosity)
As required As required As required As required As required
Cure 20 mins,
110C hot air
30 mins,
100C hot air
100-120C
hot air
100-120C
hot air
100C,
Steam
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(iii) Spreading: Spreading of latex is used in the manufacture
of proofed fabrics, which
consists of applying a suitable latex compound on the fabric
with the help of a Doctors Knife. This process has found wide
application in the backing of tufted carpets in which the
loosely woven piles of wool or jute fibres must be anchored
strongly to the base by using a
suitable compound. A compound found satisfactory in carpet
backing application is given in
Table 1.
(iv) Spraying: The adhesive property of latex has been utilized
in the spraying process for
bonding paper, cloth, leather, fibre etc. Spraying of latex is
now days largely used in the
manufacture of cushions and mattresses from latex treated coir.
Coconut fibres can be
bonded by spraying a suitable latex compound to yield latex
treated coir, which is a cheap but
useful as upholstery material. The process consists of spraying
the loose fibres with the latex
compound, drying the product, compressing the dried mass in a
mould to obtain a desired
shape and curing it in an air oven for the permanence of shape.
A typical formulation of a
latex compound suitable for spraying is given in Table 1.
(v) Foaming: The production of latex foam for mattresses and
upholstery is the most
important of all the latex processes. Latex foam is a flexible
cellular material containing
many cells (either open, closed or both) distributed throughout
the mass. There are currently
two methods of producing latex foam: the Dunlop process and the
Talalay process.
In the Dunlop process, sodium silicofluoride is used as the
gelling agent. The latex
compound is mechanically beaten and / or air blown through it to
foam. Then the requisite
amount of a dispersion of sodium silicofluoride is added, which
in presence of zinc oxide sets
the foam into gel in a mould (usually made of aluminium) in
which it is poured. The gelled
foam is then vulcanized in steam, stripped from the mould,
washed and dried. In the
compound a secondary gelling agent, Diphenyl guanidine (DPG), is
added to reduce the
gelling time so that no premature foam collapse may occur. A
typical formulation of latex
foam is given in Table 1.
In the Talalay process, partially foamed latex is poured into a
mould which is sealed and
vacuum is applied so that the foam expands to fill the mould
completely. The foam is then
frozen by cooling the mould to 35C. Carbon dioxide is then
admitted which penetrates the structure and owing to the pH change,
causes gelling. The final stage is heating of the mould
to vulcanizing temperature to complete the cure. In spite of the
high capital cost, this process
is currently used because of the excellent quality of the
product and the low rejection rate.
Dry Rubber Technology
A variety of coagulation methods are available to prepare the
rubber for dry rubber
technology processes. Since the properties of the rubber are
affected by trace ingredients and
by the coagulating agents used, rubbers of different properties
are obtained by using the
different methods. The major types of raw rubbers are:
(i) Ribbed Smoke Sheet (RSS): It is the sheet of coagulum
obtained by vertically inserting
aluminum partitions into the coagulation tanks containing the
latex and the coagulation is
effected by adding acetic acid. The sheet is then passed through
a series of mill rolls, the last
pair of which are ribbed, giving the surface of rubber a diamond
pattern, which shortens the
drying time of rubber. The sheet is then dried slowly in a smoke
house at a temperature gradient of 43-60C for about four days. The
rubber is dark in color.
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(ii) Pale Crepe: This is a premium grade of rubber, for use
where lightness of color is
important as in white side walls of tires, surgical goods etc.
For pale crepe high quality of
latex is used and the lightest colors are obtained by removing a
colored impurity, -carotene,
by a two stage coagulation process, followed by bleaching the
latex with xylyl mercaptan and
adding sodium bisulphite to inhibit an enzyme catalyzed
darkening process. The coagulum is
machined eight or nine times between grooved differential-speed
rollers with liberal washing.
(iii) Comminuted and other new process rubbers: In these cases
the coagulum is broken up
and then dried. The rubber is then packed in flat bales similar
in size to those used for major
synthetic rubbers (70-75 lbs) unlike the heavier square bales
used with smoke sheet and crepe
rubbers.
Properties of raw natural rubber
The better types and grades of natural rubber contain at least
90% of the hydrocarbon cis-1,4
polyisoprene, in admixture with naturally occurring resins,
proteins, sugars etc. The raw
material of commerce (sheet, crepe etc) comprises a molecular
weight mainly in the range of
5,00,000 to 10,00,000 which is very high for its processing.
Hence rubber has to be extensively masticated on a mill or in an
internal mixer to break down
the molecule to a size that enables them to flow without undue
difficulty when processing by
extrusion or other shaping operations. The break down occurs
more rapidly at either high
(120-140C) or moderately low (30-50C) temperature than it does
at temperatures around
100C. It is now recognized that breakdown at the more elevated
temperatures is due to
oxidative scission and that at low temperatures due to
mechanical ruptures of primary bonds;
the free radicals thus produced get stabilized by addition of
oxygen.
Because of its highly regular structure, natural rubber is
capable of crystallization, which is
substantially increased by stretching of the rubber causing
molecular alignment. This
crystallization has a reinforcing effect giving strong gum stock
(unfilled) vulcanizates. It also
has a marked influence on many other mechanical properties.
The outstanding strength of natural rubber has maintained its
position as the preferred
material in many engineering applications. It has a long fatigue
life, good creep and stress
relaxation resistance and is low cost. Other than for thin
sections, it can be used to
approximately 100C and sometimes above. It can maintain
flexibility down to 60C if compounded for the purpose. The low
hysterisis (heat generation under dynamic condition) and its
natural tack make natural rubber ideal for use in tire building.
Its chief disadvantage
is its poor oil resistance and its lack of resistance to oxygen
and ozone, although these latter
disadvantages can be ameliorated by chemical protection. Natural
rubber is generally
vulcanized using accelerated sulphur system. Peroxides are also
occasionally used,
particularly where freedom from staining by metals such as
copper is important.
Natural rubber is mainly used in passenger tires, primarily for
carcasses and white side walls,
the remainder of the tire usage is in racing cars, airplanes,
heavy duty trucks and buses,
tractors and farm vehicles. Besides, it is used in footwear
soles, industrial products such as
pump coupling, rail pads, bridge bearings, conveyor belts (cover
and friction), hoses etc.
Some typical NR formulations for use in tire and other
industrial products are given in Table
2.
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Table 2: Typical NR formulations for use in tire and other
industrial products
Ingredient Truck Trade
(normal)
Truck
Carcass
Conveyor
Belt Cover
Bridge
Bearings
Rail Pads
Natural Rubber 100 100 100 100 100
Process Oil - - 4 2 3
Pine tar - 3 - - -
Stearic Acid 2.5 2 2 1 2
Zinc Oxide 3.5 5 5 10 5
Antioxidant 2 2 2 1 1
Antiozonant - - - 4 -
ISAF Black 50 - - - -
HAF Black - - 45 - -
FEF Black - 10 - - -
MT Black - - - 35 60
SRF Black - 15 - 35 -
China Clay - - - - 20
Paraffin Wax - - 1 - 1
Accelerator
(CBS)
0.8 0.5 0.5 0.7 1
Sulphur 2 2.5 2.5 2.5 2.5
Cure 15 min @
158C
25 min @
153C
20 min @
153C
20 min @
140C
15 min @
153C
Tensile strength,
psi
4200 3800 4575 3050 2880
% Elongation 620 600 575 520 540
300% Modulus,
Psi
1440 900 1650 480 510
Shore-A
Hardness
59 50 60 60 66
Crescent Tear,
lb/in
650 350 600 - -
Synthetic Rubber
Introduction Prior to World War II, developments were being
actively pursued in Germany in the
production of a polymer as a replacement for the natural rubber
i.e. for general-purpose
application. Through commercial contacts between German and
American manufacturers,
much detail of these materials and their manufacture was known
in the USA. Hence as a
wartime necessity to make up for the deficiency of natural
rubber supplies to the allies, large-
scale manufacture of the styrene-butadiene polymers with a 25%
styrene and 75% butadine
content in USA began.
Since then a series of synthetic elastomers, both general
purpose and special purpose came
into market. Special purpose rubbers are those produced in much
smaller quantities and
having a different degree of oil and solvent resistance and / or
heat resistance from those in
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11
the general-purpose class, which are produced in large
quantities to supplement and replace
natural rubber with which they are comparable in
non-oil-resistant properties. Initially
developed special purpose rubbers are neoprene and
acrylonitrile-butadiene rubbers, which
remain the workhorses because of their cost and their oil
resistance. The market for neoprene
rubbers has been much widened by the exploitation of their
excellent resistance to ozone and
weather, and by their use in fire-resistant application such as
cable sheathing and conveyor
belting for mines. The largest outlets for nitrile rubbers are
in the engineering industries for
oil seals, O-rings, gaskets and fuel & oil hoses. Later on
chlorosulphonated polyethylene
rubbers were developed and established for applications where
solvent, chemical, ozone and
weathering resistance are required.
Fluorocarbon rubbers, with inferior low temperature properties
to the nitrile rubber but
superior oil and heat resistance, represent improvements, which
have been acceptable in the
aircraft and automobile industries. The high price of
fluorocarbon rubber and silicone
rubbers restricts their widespread use even though silicone
rubbers are unique in their wide
range of service temperature.
Polyurethane rubbers possess certain outstanding properties.
They can have higher tensile
strengths than any other rubber, excellent tear and abrasion
resistance, and outstanding
resistance to ozone, oxygen and aliphatic hydrocarbons.
The thermoplastic elastomers are a unique new class of polymers
in which the end use
properties of vulcanized elastomers are combined with the
processing advantages of
thermoplastics. These polymers yield useful articles having true
elastomeric properties
without compounding or vulcanization.
Hence, it is apparent that rubber compounders have now a wide
spectrum of elastomers to
choose from, to meet one or more of the requirements for
specific end use.
Styrene Butadiene Rubber (SBR) SBR is the highest volume and
most important general-purpose synthetic rubber in the entire
world. Although it was of poor quality in many respects to
natural rubber, it has achieved a
high market penetration on account of three factors:
- Its low cost
- Its suitability for passenger car tires, particularly because
of its good abrasion
resistance
- A higher level of product uniformity than that can be achieved
with natural rubber.
Composition and Structure: SBR is a copolymer of styrene (CH2 CH
C6H5) and 1,3-
butadiene (CH2 CH CH CH2). With the exception of some special
grades, typically
the styrene content is 23.5% by weight, which corresponds to one
styrene to six or seven
butadiene molecules per chain. The monomers are randomly
arranged in the chain.
Manufacture: SBR can be produced either by emulsion
polymerization or by solution
polymerization technique.
Emulsion SBR: The monomers, styrene and butadiene taken in the
weight ratio of about 1:3,
are emulsified in deionized water using soap as emulsifier. The
polymerization reaction is
carried out at about 50C (hot SBR grades) or at about 4C (cold
SBR grade). The chain reaction is initiated by decomposition of
peroxide or a peroxy disulfate into free radicals in
-
12
case of hot SBR and by a hydroperoxide/ferrous sulphate redox
system in case of cold SBR.
Dodecyl mercaptan is used as a chain transfer agent or modifier
to control the toughness of
the product which otherwise may limit its processibility.
Typical formulations of hot and
cold SBR are given in Table 3.
Table 3: Typical formulations of hot and cold SBR
Ingredient Hot SBR Cold SBR
Butadiene 75.0 72.0
Styrene 25.0 28.0
Dodecyl Mercaptan 0.5 0.2
Potassium peroxydisulfate 0.3 -
Diisopropyl benzene
hydroperoxide
- 0.08
Ferrous sulphate (FeSO4, 7H2O) - 0.14
Potassium pyrophosphate
(K4P2O7)
- 0.18
Soap Flakes 5.0 -
Rosin Acid Soap - 4.0
Deionized Water 180.0 180.0
In hot SBR, polymerization is stopped at 70-75% conversion by
adding a short stop (0.1 part
hydroquinone) whereas in case of cold SBR, it is stopped at 60%
conversion to control its
molecular weight. After the addition of an antioxidant (1.25
parts of N-phenyl -
napthylamine), the latex is coagulated by the addition of brine
and dilute sulphuric acid. The
coagulated crumb is washed, dried and baled for shipment.
The cold SBR has a more linear molecular structure and imparts
vulcanizates much improved
properties than hot SBR. Other improvements directed towards
specific end uses include:
- The development of oil extended SBR in which a rubbery polymer
of very high
molecular weight is blended with substantial amounts of
hydrocarbon oil. This
provides a lower cost alternative to a polymer of conventional
average molecular
weight.
- Preparation of carbon black master batches of regular and oil
extended cold SBR.
These are of interest to rubber manufacturers having limited
mixing capability and
those who wish to avoid handling of loose black in factory.
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13
Solution SBR: Several solution SBRs are offered commercially.
The random copolymers
are rubbery and like emulsion SBR but with several improved
properties. The random
products have narrower molecular weight distribution, less chain
branching, higher cis
content, lighter color and less non - rubber constituents than
the emulsion SBRs. As a result,
they are reported to have better abrasion resistance, better
flex, higher resilience and lower
heat build-up than the emulsion rubber; tensile, modulus,
elongation and cost are comparable.
Polymerization of styrene and butadiene is usually carried out
with an alkyl lithium type
catalyst in a non-polar solvent. In general, continuous reactor
system is used. As the
polymerized solution (cement) leaves the last reactor, stopper
and stabilizer are added. The
cement is steam stripped to get rubber crumb and to recover the
solvent; un-reacted
monomers are recycled. The rubber crumb is dried on tray or
extruder drier.
Properties: Like NR, SBR is an unsaturated hydrocarbon polymer.
Hence un-vulcanized
compound will dissolve in most hydrocarbon solvents whilst
vulcanized stocks will swell
extensively. A major difference between SBR and natural rubber
is that SBR does not break
down to a great extent on mastication. SBR is supplied at a
viscosity considered to provide
the balance of good filler dispersibility and easy flow in
processing equipment. The
processing behaviour of SBR, however, is not as good as natural
rubber in many other
respects. Mill mixing is generally more difficult; it has lower
green strength (i.e. inferior
mechanical properties in the un-vulcanized state) and does not
exhibit the natural tack, which
is essential in plying together or otherwise assembling pieces
of unvulcanized rubber.
Whereas natural rubber is crystalline with a Tm of about 50C,
SBR is amorphous due to its
molecular irregularity. Natural rubber crystallizes on extension
at ambient temperatures to
give a good tensile strength even with gum stocks. Gum
vulcanizates of SBR on the other
hand are weak and it is essential to use reinforcing fillers
such as fine carbon blacks to obtain
products of high strength. Black reinforced SBR compounds
exhibit very good abrasion
resistance, superior to corresponding black reinforced NR
vulcanizates at temperatures about
14C. Against this however, the SBR vulcanizates have lower
resilience, fatigue resistance
and resistance to tearing and cut growth. With their lower
un-saturation, SBR also has better
heat resistance and better heat ageing qualities. SBR extrusions
are smoother and maintain
their form better than those of NR.
Compounding: For many uses, blends of SBR and other rubber such
as NR or cis -
polybutadiene are made. Compounding recipes should be
proportioned to balance the
requirements for each type of rubber used. All types of SBR use
the same basic
compounding recipes, as do other un-saturated hydrocarbon
polymers. They need sulphur,
accelerators, antioxidants (and antiozonants), activators,
fillers, and softeners or extenders.
SBR requires less sulphur than NR for curing, the usual range
being 1.5-2.0 phr. of sulphur
based on rubber hydrocarbon. All styrene-butadiene rubbers
because of their lower
unsaturation, are slower curing than natural rubber and require
more acceleration. Zinc
stearate (or zinc oxide plus stearic acid) is the most common
activator for SBR. Recipes may
also contain plasticizers, tackifiers, softeners, waxes, reclaim
etc.
Processing of SBR compounds is similar to that of natural (or
other) rubber. The ingredients
are mixed in internal mixers or on mills, and may then be
extruded, calendered, molded and
cured in conventional equipment. In general, the rubber, zinc
oxide, antioxidant and stearic
acid are mixed; then carbon black is added in portions, with the
softener or oil. This may be
considered as masterbatch. It may be desirable at this point to
dump, sheet out and cool the
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14
CH2 CH CH CH2
1,3 Butadiene
CH2
C C
CH2
Cis 1, 4
CH2 CH
CH CH2
1,2 (or vinyl )
CH2
C C
CH2
Trans 1,4
H H
H
H
batch. In the second phase, all the ingredients are mixed, with
sulphur and accelerator being
added last and mixing is continued till sulphur is well
dispersed.
Applications: While passenger tires and tire products account
for the major portion of SBR
consumption, a wide variety of other products are also
fabricated from this rubber where its
low cost coupled with adequate physical properties lead to its
preference over more
expensive materials, particularly natural rubber. SBR finds uses
in mechanical goods,
footwear, belting, hose, tubing, wires and cables, adhesives,
latex goods etc.
Polybutadiene
Polybutadiene was first prepared during World War I by metallic
sodium catalyzed
polymerization of butadiene as a substitute for natural rubber.
However, polymer prepared
by this method and later by free radical emulsion polymerization
technique did not possess
the desirable properties for its applications as a useful
rubber. With the development of the
Ziegler- Natta catalyst systems in the 1950s, it was possible to
produce polymers with a
controlled stereo regularity, some of which had useful
properties as elastomers.
One distinguishing feature of polybutadiene is its
microstructure, i.e. the ratio of cis, trans
and vinyl configuration. Polymers containing 90-98% of a cis-1,4
structure can be produced
by solution polymerization using Zeigler- Natta catalyst systems
based on titanium, cobalt or
nickel compounds in conjunction with reducing agents such as
aluminum alkyls or alkyl
halides. Useful rubbers many also be obtained from medium cis-
polybutadiene (44% cis content) using alkyl lithium as catalyst in
solution polymerization.
`
Today commercial polybutadienes are made exclusively by solution
polymerization
processes employing organometallic catalysts capable of
controlled microstructure, molecular
weight distribution and branching. Solution polymers are
characterized by fairly narrow
molecular weight distribution and less branching than emulsion
butadiene, which account for
some of the major differences in processing and performance.
Manufacture: A polybutadine with high cis content is obtained by
using a titanium catalyst
containing iodine, e.g., the combinations of trialkyl aluminium
compound such as tri-isobutyl
aluminium and titanium tetraiodide, or an alkylaluminium, iodine
and titanium tetrachloride.
Aromatic and aliphatic solvents can be used for high cis-1,4
polymer at 0-70C. A typical
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15
polymerization recipe yielding 90% in 2 hours at 50C is, benzene
85 ml; butadiene 15 ml; (i-
C4H9)3 Al 50 mg and TiI4 25 mg. The catalyst composition greatly
influences activity.
Use of organolithium compounds such as butyl lithium in heptane,
produces butadiene
polymers in a reproducible manner because of their solubility in
hydrocarbon and thermal
stability. Alkyl lithium initiation takes place in a homogeneous
reaction mixture with a
complete absence of termination and other side reactions,
thereby giving living polymers. This fact, along with the ability
to propagate other monomers and the ability of polar solvents
to modify the reactivity and microstructure of polybutadiene,
allows a great deal of flexibility
that is not offered by free radical, coordination or cationic
mechanisms.
Properties: The structure of cis-1,4 polybutadiene is very
similar to that of the natural rubber
molecule. Both the polymers are unsaturated hydrocarbons but,
whereas with natural rubber
molecule the double bond is activated by the presence of a
methyl group, the polybutadiene
molecule, which contains no such group, is generally somewhat
less reactive. Further more,
since the methyl side group tends to stiffen the polymer chain,
the glass transmission
temperature of polybutadiene (-70 to 100C) is consequently less
than that of natural rubber molecule. This lower Tg has a number of
ramifications on the properties of
polybutadiene. For example, at room temperature, polybutadiene
compounds generally have
higher resilience than similar natural rubber compounds. In turn
this means that the
polybutadiene rubbers have a lower heat build-up and this is
important in tire application. On
the other hand, these rubbers have poor tear resistance, poor
tack and poor tensile strength.
For this reason, polybutadiene rubbers are seldom used on their
own but more commonly in
conjunction with other elastomers. For example, they are blended
with natural rubber in the
manufacture of truck tires and, widely with SBR in the
manufacture of passenger car tires.
Their use also improves tread wear.
Processing : Most polybutadiene rubbers possess inherently high
resistance in breakdown
and poor mill banding characteristics. At temperature below 100
to 110F the rubber is
continuous on the mill rolls, glossy and smooth in appearance,
and bands tightly. As the
temperature of the stock is increased, the band becomes rough
& loose on the mill and loses
cohesion so that the milling is poor. It normally displays very
little breakdown as a result of
intensive mixing. However, polybutadiene can be broken down with
certain peptizers such
as modified zinc salt of pentachlorothiophenol and
diortho-benzamidophenyl disulfide to
obtain some improvement in processing.
Blends of cis-polybutadiene and natural rubber were made
initially as a means of obtaining
improved processing characteristics. It was then noted that
polybutadiene rubber conferred
many of its desirable properties such as a high tolerance for
extender oil, excellent abrasion
resistance and outstanding hysteresis properties to the blends,
e.g. blends of polybutadiene
rubber with clear and oil extended SBR or oil black masterbatch
are easily prepared with high
tolerance for carbon black and oil levels.
Polybutadiene rubbers are usually vulcanized with sulphur and
accelerator whether used
alone or in blends. Polybutadiene- natural rubber blends having
a useful balance of physical
properties can be obtained with a wide range in sulphur levels
(1.0 to 2.5 phr) and appropriate
accelerator levels (0.6-1.2 phr) to get the best balance in
properties.
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16
x y
Applications: Polybutadiene rubber has been primarily used both
in passenger and heavy
duty trucks tires as blends with natural rubber and SBR taking
advantages of its inherently
good hysteresis properties, abrasion resistance and crack growth
resistance.
Significant amount of polybutadiene is used in footwear and
belting compounds as a means
of improving abrasion and durability. The outstanding resilience
or abrasion resistance of the
polymer has been utilized in the manufacture of solid golf balls
and high rebound toy balls
and shock absorber. Polybutadiene as well as butadiene styrene
rubbers are used extensively as modifier of styrene to make high
impact polystyrene.
Nitrile Rubber (NBR)
In the course of work on the copolymerization of 1,3 - butadiene
with mono-olefins, Konrad and co-workers (1930) obtained a
synthetic rubber based on butadiene and acrylonitrile
which when vulcanized had excellent resistance to oil and petrol
classifying it as a special
purpose rubber. Pilot plant production of Buna N, as this
product was first named, started in
Germany in 1934 and full-scale production started in 1937 by
Farbenfabriken Bayer AG
(Germany) with a trade name PERBUNAN. The polymerization
reaction can be written as: CN CN
CH2 = CH CH=CH2 + CH2 = CH CH2 CH = CH- CH2 CH2 CH
The acrylonitrile content of the commercial rubbers ranges from
25 to 50% with 34% being a
common and typical value.
Manufacture: Basically, nitrile rubbers are manufactured by
emulsion copolymerization of
butadiene and acrylonitrile. As the ratio of butadiene to
acrylonitrile in the polymer largely
controls its properties, the design of the polymerization recipe
and the temperature at which
this is carried out are important features of nitrile rubber
production. The nature and amount
of modifiers also influence the properties of the end
product.
The early nitrile rubbers were all polymerized at about 25-50C
and these hot polymers contain a degree of branching in the polymer
chain known as gel. By analogy with the developments in the
emulsion polymerization of SBR, since early 1950s, an
increasing
number of nitrile rubbers are being produced by cold
polymerization at about 5C. This results in more linear polymers
containing little or no gel which are easier to process than
hot polymers. The dry rubber is obtained by coagulation of
emulsion with salts and acids into fine crumbs. The pH of the
slurry is adjusted with caustic solution and it is then
filtered,
washed, denatured and dried.
Properties: Acrylonitrile imparts very good hydrocarbon oil and
petrol resistance to the
polymer. As a general rule, raising the acrylonitrile level
increases the compatibility with
polar plastics such as PVC, slightly increases tensile strength,
hardness and abrasion
resistance and also enables easier processing; however, in the
process, low temperature
flexibility and resilience properties deteriorate. At
temperatures up to 100C or with special
compounding up to 120C, nitrile rubber provides an economic
material having a high
resistance to aliphatic hydrocarbon oils and fuels. It has
limited weathering resistance and
poor aromatic oil resistance. It can generally be used down to
about 30C, but special grades can operate at still lower
temperatures.
1,3 -Butadiene Acrylonitrile Nitrile rubber
-
17
Generally NBR possesses better heat resistance than neoprene,
but like natural rubber, is
subject to ozone cracking. Products with low compression set
properties can be made. The
physical properties of nitrile rubbers are good when the rubbers
are compounded with carbon
black of suitable type, mainly the semi reinforcing type though
unfilled vulcanizates have
very low tensile strength.
In general NBR is compounded along lines similar to those
practiced with natural rubber and
SBR. The rubbers may be vulcanized by the conventional
accelerated sulphur systems and
also by peroxides. The use of tetramethyl thiuram disulphide
without sulphur or tetramethyl
thiuram monosulphide with sulphur generally produces
vulcanizates with the lower
compression set properties. A tetramethyl thiuram monosulphide
sulphur cure is an excellent general-purpose system. Another widely
used general-purpose cure system is 1.5
MBTS/ 1.5 sulphur; for improved ageing 3 MBTS / 0.5 sulphur is
recommended. When NBR
is blended with PVC, products with improved resistance to ozone
and weathering, gloss,
bright colors, abrasion & oil resistance, and flame
resistance are obtained when used
alongwith suitable plasticizers.
Applications: Polymers with high acrylonitrile content are used
where the utmost oil
resistance is required such as oil well parts, fuel cell liners,
fuel hose and other applications
requiring resistance to aromatic fuels, oils and solvents. The
medium grades are used in
applications where the oil is of lower aromatic content such as
in petrol hose and seals. The
low and medium low acrylonitrile grades are used in case where
low temperature flexibility is
of greater importance than oil resistance.
Neoprene Rubber (CR)
Neoprene is the generic name for chloroprene polymers
(2-chloro-1,3 butadiene)
manufactured since 1931 by E.I. DuPont de Nemours and company.
Today these materials
are amongst the leading special purpose rubbers (i.e. non tire
rubbers).
The solid neoprenes are classified as general purpose, adhesive
or specialty types. General
purpose types are used in a variety of elastomeric applications
particularly molded and extruded goods, hose, belts, wire and
cable, heels and soles, tires, coated fabrics and gaskets.
The adhesive types are adaptable to the manufacture of quick
setting and high bond strength
adhesives. Specialty types have unique properties such as
exceptionally low viscosity, high
oil resistance or extreme toughness. These properties make
specialty neoprenes useful in
unusual applications: for example, crepe soles, prosthetic
applications, high solids cements
for protective coatings in tanks and turbines. Neoprenes are
also available in latex form,
which like dry rubbers may be classified as general purpose and
specialty types.
Manufacture: Neoprene rubbers are manufactured by polymerizing
2-chloro-1,3 butadiene
by free radical emulsion polymerization technique at 40C using
an initiator such as
potassium persulphate, emulsifiers, modifiers such as dodecyl
mercaptan and stabilizers. A
sulfur- modified grade such as Neoprene GN is the oldest
general-purpose neoprene still
produced today. The manufacturing process for neoprene GN is
typical of a commercial
emulsion polymerization system. A solution of sulfur and rosin
in chloroprene is emulsified
with an aqueous solution of caustic soda and the sodium salt of
naphthalene sulfonic acid-
formaldehyde condensation product. The sodium rosin soap
emulsifier is formed in situ; the
condensation product is used to stabilize the latex till it is
subsequently acidified for polymer
isolation. The polymer chain is built up through the addition of
the monomer units, of which
approximately 98% add in the 1,4 - positions. About 1.5%
additions in 1,2 - positions are
-
18
utilized in the vulcanization process since in this arrangement
the chlorine atom is both
tertiary and allylic. Accordingly, it is strongly activated and
thus becomes a curing site on
the polymer chain.
Cl Cl
~~CH2 C = CH CH2~~ ~~ CH2 C ~~
1,4 - addition CH
CH2
1,2 - addition
Properties: Since neoprene predominantly consists of 1,4 - trans
unit, both the raw and cured
polymer crystallize, particularly upon stretching. Neoprene
vulcanizates give high tensile
strength owning to stress induced crystallization.
Crystallization rate is reduced by
modification of the polymers molecular structure and / or
incorporation of a second monomer in the polymerization reaction.
The commercial polymers have a Tg of about -
43C and a Tm of about 45C so that at usual ambient temperatures
the rubber exhibits a
measure of crystallinity.
The close structural similarities between neoprene and the
natural rubber molecule are
apparent. However, whilst the methyl group activates the double
bond in the polyisoprene
molecule, the chlorine atom exerts opposite effect in neoprene.
Thus the polymer is less
liable to oxygen and ozone attack. The chlorine atom has two
other positive impacts on the
polymer properties. Firstly, the polymer shows improved
resistance to oil compared with all
hydrocarbon rubbers and these rubbers also have a measure of
resistance to burning which
may further be improved by use of fire retardants. These
features together with a somewhat
better heat resistance than the diene hydrocarbon rubbers have
resulted in the extensive use of
these rubbers over many years.
Pure gum vulcanizates of CR, like those of natural rubber show
high levels of tensile
strength. However, to provide optimum processing
characteristics, hardness and durability,
the majority of the neoprene compounds contain fillers. This
rubber in general has a good
balance of mechanical properties and fatigue resistance second
only to natural rubber, but
with superior chemical, oil and heat resistance. Hence, It is
widely used in general
engineering applications. It is suitable for use with mineral
oils and greases, dilute acids and
alkalis, but are unsuitable in contact with fuels. It has
generally poorer set and creep than
natural rubber.
It is less resistant than natural rubber to low temperature
stiffening but can be compounded to
give improved low temperature resistance. It has good ozone
resistance. Service in air is
satisfactory up to 85-90C with suitable antioxidant. Neoprene
vulcanizates show a high
level of resistance to flex cracking. The resilience of a pure
gum neoprene vulcanizate is less
than that of a similar natural rubber compound. However,
increase in filler loading has lesser
influence on the consequent decrease in resilience, as a result
of which, the resilience of most
practical neoprene is above than that of natural rubber with
similar filler loading.
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19
x y
Compounding of Neoprene: Neoprene products require certain
engineering properties
usually associated with strength or working environment. Raw
neoprene is converted to
these products by mixing selected ingredients into the neoprene
and curing the resulting
compound.
Metal oxides are essential in vulcanizate curing systems, the
best system being a combination
of magnesium oxide and zinc oxide. This combined metal oxide
system provides the most
desirable relation of process safety to rate and state of cure
combined with vulcanizate quality
and age resistance. Neoprene may be vulcanized with sulphur, but
metal oxides must also be
present. The reaction is much slower than that of natural rubber
or copolymers of butadiene.
Cross-linking with sulphur probably occurs at the double bonds
in the linear polymer chain
rather than at the allylic position. Though it is impossible to
designate a base compound
meeting all requirements, a starting formula for general purpose
neoprenes could be, neoprene 100/ antioxidant 2/ magnesium oxide
1-4/ zinc oxide 5/ accelerator and / or curing
agent 0-3. In all operations it is important to avoid pre-cure
or scorching as a result of too
much heat history. This means short mixing cycles at the minimum
possible temperatures.
Accordingly, mixing cycles call for processing aids,
stabilizers, antioxidants, magnesia,
fillers with softeners, and finally, zinc oxide with
accelerators and / or curing agents.
Applications: Application and end products of polychloroprene
are probably much more
than any other specialty synthetic rubber. Some of the more
important uses are in adhesives,
transport sector, wire and cable, construction, hose and
belting.
There are hundreds of different kinds of neoprene-based
adhesives available for use in shoes,
aircraft, automobiles, furniture, building products and
industrial components. In the
automotive field, neoprene is used to make window gaskets,
V-belts, sponge door gaskets,
wire jackets, molded seals, motor mounts etc. In aviation, it is
used in mountings, wire and
cable, gaskets, deicers, seals etc. In railroads, it is used in
track mounting, car body
mountings, air brake hose, flexible car connectors etc. In wire
and cable, jackets for
electrical conductors are one of the oldest uses. In
construction, neoprene is used in highway
joint seals, bridge mounts, pipe gaskets, high-rise window wall
seals and roof coatings. All
types of hoses including industrial and automotive, garden, oil
suction, fire, gasoline curb
pump, oil delivery and air hoses are made from neoprene.
Neoprenes heat and flex resistance make it an excellent choice for
making V-belts, transmission belts, conveyor belts and
escalator handrails.
Ethylene- Propylene Rubber
Ethylene propylene rubber was first introduced in the United
States, in limited commercial quantities in 1962. Though full-scale
commercial production only began in 1963, ethylene-
propylene rubber is one of the fastest growing polymers today
because of its certain unique
properties. These poly olefins are produced in two main types:
the standard binary
copolymers (EPM) and unsaturated ternary copolymers (EPDM).
A fully saturated copolymer of ethylene and propylene (EPM) is
having the following
structure:
CH3
CH2 CH2 CH2 CH
EPM copolymer (x/y = 50/50 to 65/35).
-
20
CH2 CH2
Predominant structure present in the terpolymer
CH2
CH CH3
CH2
Because of their saturated structure, the raw polymer could not
be vulcanized using
accelerated sulphur systems and the less convenient peroxide
curing systems were required
causing reluctance for the wholehearted acceptance by the rubber
processors. Besides,
peroxide curing systems are much more liable to premature
vulcanization (scorch) than
accelerated sulphur systems which can lead to high scrap
generation.
As a consequence, a third monomer, a non-conjugated diene is
introduced in the EPM
backbone in small quantity (3-8%), which provided crosslink
sites for enabling it to be
vulcanized with accelerated sulphur vulcanization. Such
ethylene- propylene-diene ternary
copolymers are designated as EPDM rubber.
The EPDM rubbers, whilst being a hydrocarbon, differ
significantly from the diene
hydrocarbon rubbers in two principal ways:
i) The level of un-saturation is much lower, giving the rubber a
much better heat,
oxygen and ozone resistance.
ii) The dienes used are such that the double bonds in the
polymer are either on a side chain or as part of a ring in the main
chain. Hence should the double bond become
broken, the main chain will remain substantially intact. Until
some years ago
dicyclopentadiene (DCPD) was mostly used, but these rubbers are
slow curing and
therefore, cannot be co-cured with diene rubbers. The recent
trend is towards faster
curing grades, and most companies now incorporate ethylidene
norbornene (ENB) as
the third monomer. Some typical dienes used as third monomer in
ethylene -
propylene rubbers are given in table 4.
Table 4: Typical dienes used in ethylene - propylene rubbers
Manufacture : The monomers ethylene and propylene are
copolymerized in solution in
hexane using Ziegler-Natta type catalysts such as vanadium
oxychloride (VOCl3) and an
alkyl aluminium or an alkyl aluminium halide (e.g. Al (C2H5)2
Cl). The ratio in which the
CH CH
Monomer
Dicyclopentadiene (DCPD)
CH CH
CH CH3 Ethylidene norbornene (ENB)
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21
CH3
H2C C
CH3
C CH CH2
Isobutylene
monomers are polymerized does not depend on the ratio in which
they are taken for reaction
but on the nature of the catalyst. The polymerization is highly
exothermic (1100 btu/lb). The
heat is constantly removed to maintain the polymerization
temperature at 100F to ensure a
product with desired average molecular weight and
distribution.
Properties: The ethylene propylene rubbers are predominantly
amorphous and non-
stereoregular, and therefore, the pure gum vulcanizates show low
tensile strength. Whereas
butyl elastomers are highly damping at ambient temperatures, the
poly olefin elastomers are
highly resilient.
The most striking features amongst the properties of the
vulcanizates are the excellent
resistance to atmospheric ageing, oxygen and ozone upto 150C.
Probably it is the most water resistant rubber available and the
resistance is maintained to high temperatures (upto
180C in steam for peroxide cures). The highest temperature
resistance is achieved by using
peroxide cure. It has good resistance to most water based
chemicals and vegetable oil based
hydraulic oils. However, it has very poor resistance to mineral
oils and diester based
lubricants.
EPM can be cured with peroxides such as dicumyl peroxide. EPDM,
the unsaturated
polymers can be cured using sulphur and common rubber
accelerators such as tetramethyl
thiuram disulphide (TMTDS) activated with mercaptobenzothiazole
(MBT). A faster curing
can be achieved by activating with a dithiocarbamate such as
zinc dibutyl dithiocarbamate
(ZDBDC). EPDM compounds generally carry high loading of oils
such as paraffinic and
napthenic oils without too much loss in vulcanizate properties.
In order to get good
properties, the use of reinforcing black or white filler is
recommended.
Applications: The tire related end use of EPDM is as an additive
to the diene rubber (SBR, natural rubber) compounds in the tire
sidewalls and coverstrips to improve their resistance to
ozone and weather cracking while under stress and during
flexing; EPDM is now almost
universally used in this applications. Besides, the unique
inherent properties of olefinic
elastomers have enabled it for use in cars, domestic and
industrial equipment, hose, wire and
cable, coated fabrics, linings, footwear, rug underlay, matting
pad etc.
Butyl Rubber (IIR)
Butyl rubber has been commercially produced since 1942, and at
the present time is a well-
established specialty elastomer used in a wide range of
applications. Commercial grades of
butyl rubber are prepared by copolymerizing isobutylene with
small amounts of isoprene at
1-3% of the monomer feed.
CH3
CH2
Isoprene
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22
= quionone dioxime = dinitrosobenzene
Homopolymer from isobutylene has little use as a rubber because
of high cold flow (Tg about
73C) but the copolymer with isoprene to introduce un-saturation
for cross-linking is a useful rubber which is widely used in many
special applications.
Manufacture: The monomers are polymerized in solvents such as
methyl chloride. The
reaction is unique in that it is an extremely rapid cationic
polymerization conducted at a low
temperature (-100C) using Friedel-Crafts catalysts such as AlCl3
or BF3. The purity of
isobutylene is important for acquiring high molecular weight.
The n-butene content should
be below 0.5% and the isoprene purity should be 95% or more. The
methyl chloride solvent
and the monomer feed must be carefully dried.
Properties: Owing to the symmetric nature of the isobutylene
monomer, the polymer chains
have a very regular structure. Hence, butyl elastomers are
self-reinforcing with a high pure
gum strength (250 Kgf/cm2 ). The abundance of methyl side groups
in the chains cause a
considerable steric hindrance to elastic movements; although Tg
values of around - 65C
have been measured, the resilience of vulcanizates at ambient
temperatures is very low (about
14% rebound). On the other hand, the densely packed structure of
these elastomers causes
the gas permeability to be very low, and, because of this, for a
long time the main application
of butyl rubbers was for inner tubes of pneumatic tires. Mainly
as a result of the rather rigid
and highly saturated chains, the polymer excels in ozone and
weathering characteristics, heat
resistance, chemical resistance and abrasion resistance.
Regular butyl rubber is commercially vulcanized by three basic
methods. These are
accelerated sulphur vulcanization, cross-linking with dioxime
and dinitroso related
compounds and the resin cure.
As common with more highly unsaturated rubbers, butyl may be
crosslinked with sulphur,
activated by zinc oxide and organic accelerators. In contrast to
the higher unsaturated
varieties, however, adequate vulcanization can be achieved with
very active thiuram and
dithiocarbamate accelerators. Other less active accelerators
such as thiazole derivatives can
be used as modifiers to improve processing scorch safety. Most
curative formulation include
the following ranges of ingredients:
Ingredient Parts by Weight
Butyl Elastomer 100.0
Zinc Oxide 5.0
Sulphur 0.5-2.0
Thiurum or dithiocarbamate accelerator 1.0-3.0
Modifying thiazole accelerator 0.5-1.0
The cross linking of butyl with p-quinone dioxime or p-quinone
dioxime dibenzoate
proceeds through an oxidation step that forms the active cross
linking agent, p-
dinitrosobenzene.
HON = = NOH + [ O ] O = N N = O
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23
The use of PbO2 as the oxidizing agent results in very rapid
vulcanizations, which can
produce room temperature cure for cement applications. In dry
rubber processing, the
dioxime cure is used in butyl based electrical insulation
formulation to provide maximum
ozone resistance and moisture impermeability. Curing with
reactive phenol formaldehyde
resins results in vulcanizates with excellent ageing and heat
resistant properties.
Chlorobutyl Rubber
The introduction of a small amount of chlorine (1.2 wt.%) in the
butyl polymer gives rise to
chlorobutyl rubber, which can be blended better with
general-purpose rubbers due to
increased polarity. Moreover, in addition to the various cure
systems acting via double
bonds, a variety of new cure systems effective through the
allylic chloride can be used in
chlorobutyl rubber.
Applications: As already mentioned, the high degree of
impermeability to gases makes butyl
atmost an exclusive choice for use in inner tubes. It is of
importance in air barriers for
tubeless tires, air cushions, pneumatic springs, accumulator
bags, air bellows and the like.
A typical formulation for a butyl rubber passenger tire inner
tube is given below:
Ingredients Parts by weight
Butyl Rubber 100
GPF carbon Black 70
Paraffinic process oil 25
Zinc Oxide 5
Sulphur 2
Tetramethyl thiurum disulphide 1
Mercapto benzothiazole 0.5
Cure 5 minutes @ 177C or 8 minutes @ 165C.
The high thermal stability has found widespread use in the
expandable bladders of automatic
tire curing presses. Another application would be conveyor
belting for hot materials
handling.
The high level of ozone and weathering resistance enables butyls
to be used in rubber
sheeting for roofs and water management application. The ozone
resistance coupled with
moisture resistance of butyl rubber finds utility in high
quality electrical insulation. Due to
the delayed elastic response to deformation or damping, butyl
rubber has found wide applications in automotive suspension bumpers
and anti-vibration shock absorbing pads in
the various machines.
While butyl vulcanizates get highly swelled by hydrocarbon
solvents and oils, they are only
slightly affected by oxygenated solvents and other polar
liquids. This behavior is utilized in
elastomeric seals for hydraulic systems using synthetic fluids.
The low degree of olefinic
unsaturation in the polymer backbone imparts mineral acid
resistance to butyl rubber
composition. Immersion in 70% H2 SO4 acid for 13 weeks could
hardly affect a butyl
compound adversely.
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24
100-150C
Chlorobutyl is used as innerliners for tubeless tires, tire
sidewall components and heat
resistant truck inner tubes, hose (steam & automotive),
gaskets, conveyor belts, adhesives and
sealants, tank linings, tire curing bags, truck cab mounts,
aircraft engine mounts, rail pads,
bridge bearing pads, pharmaceutical stoppers and appliance
parts.
Polysulfide Rubber (TR)
Since the commercial introduction in 1929 of the polysulfide
polymers, they have been
utilized in specialty applications due to their excellent oil
and solvent resistance as well as
good ageing properties. Although the original polymers were
solid rubbery materials, today
the predominant product, discovered some 20 years later, is the
mercaptan terminated liquid
polymer (LP). It can be transformed in situ from a liquid state
into a solid elastomer, even at
low temperatures, which makes its use convenient for adhesives,
coatings and sealants.
Polysulfide rubbers are produced by the condensation of sodium
polysulfide with
dichloroalkanes:
R Cl2 + Na2 Sx R Sx
The polymer varies both in characters of R and x and in the
length of polysulfide chain. In the
year 1929, Thiokol Chemical Corporation, New Jersey first
introduced a polysulfide rubber
(Thiokol A) based on the reaction product of ethylene dichloride
and sodium tetrasulfide.
The different polymers produced by the Thiokol Chemical
Corporation are given in table 5:
Table 5: Various grades of polysulfide rubbers (Thiokol)
Polymer Dihalide, R X % Sulfur
Thiokol A ClCH2 CH2 Cl 4 84
Thiokol B Cl (CH2)2 OCH2O (CH2)2 Cl 4 64
Thiokol FA ClCH2 CH2 Cl
CH2 (OCH2 CH2 Cl)2
2 47
Thiokol ST CH2 (OCH2 CH2 Cl)2
2% Trichloropropane
2.2 37
Manufacture: The general method of preparation of polysulfides
is to add the dihalide
slowly to an aquous solution of sodium polysulfide. Magnesium
hydroxide is often
employed to facilitate the reaction, which takes 2-6 hours at
70C. Sodium polysulfide is
usually produced directly from sodium hydroxide and sulfur at
elevated temperature.
Properties: The solid polymers are used almost exclusively in
applications where good
resistance to solvents is required. This depends on the amount
of sulfur in the molecule.
Thiokol A is resistant to every type of organic solvent.
However, its odour, processing
characteristics and mechanical properties are very poor, and the
other types which have
moderate physical properties and better all-round solvent
resistance than neoprene or nitrile
rubbers are more widely employed. Curing agents for thiokols are
diverse, but it is
6 NaOH + 2 (x + 1) S 2 Na2Sx + Na2S2O3 + 3 H2O where x = 1 -
4
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25
customary to use an organic accelerator (e.g. MBTS or TMTD with
zinc oxide and stearic
acid for Thiokol FA). The thiol terminated polymers (Thiokol ST)
can be crosslinked by
metal oxides, metal peroxides, inorganic oxidizing agents,
peroxides and p-quiononedioxime.
Carbon black, usually SRF or FEF in 40-60 phr loading, is
essential for adequate strength.
The polysulfide rubbers have very good resistance to oils,
fuels, solvents, oxygen and ozone,
impermeable to gases but have poor mechanical properties and
poor heat resistance. They
are however, not recommended for use against strong oxidizing
acids in any concentration.
They are blended with other synthetic rubbers for improved
processing.
Applications: Because of their excellent oil and solvent
resistance and impermeability to
gases, polysulfides find applications in specialty areas.
Thiokol FA is used in the
manufacture of rollers for can lacquering, quick drying printing
ink application and grain
coating of paint on metals. Another major application of Thiokol
FA is in solvent hose liner.
Type ST is used in the Gas Metal Diaphragms. Primary use for
type A is as flexibilizer for
sulfur. 2-5 parts of Thiokol A dissolved in molten sulfur
prevents it from crystallization so
that it can be used as a mortar for acid pickling tanks, water
sewers and oil pipes. Several
applications for polysulfide rubber are, as linings and sealants
in airplane fuel tanks, concrete
fuel storage tank linings, tank car linings, self-sealing
aircraft tanks and deicer on wings.
Liquid Polysulfides
A series of liquid thiol terminated polymers (Thiokol LP
2,3,4,31, 32 & 33) are available
based on the diethylene formal disulfide structure but
containing some branching, and
Thiokol LP 205 based on dibuthylene formal disulfide. These low
molecular weight
polymers are formed by reductive cleavage of disulfide linkage
in solid rubber by means of a
mixture of sodium hydrosulfide and sodium sulfite. The reaction
is carried out in water
dispersion and the relative amount of the hydrosulfide and
sulfite controls the extent of
cleavage and liquid polymers of varying molecular weights can be
readily prepared. The
sodium hydrosulfide splits a disulfide link to form a thiol and
a sodium salt of thiol. The
extra sulfur atom is taken up by sodium sulfide.
The sodium salt of the polysulfide is converted back to the free
thiol on coagulation with
acid.
While the commercial liquid polymers contain terminal thiol
groups produced by the above
method, liquid polymers have been prepared experimentally with
terminal alkyl, aryl,
hydroxyl, allyl and carboxyl groups. These materials can be
produced by using a mixture of
dihalide with the appropriate monohalide in the initial reaction
with sodium polysulfide. The
molecular weight of the product is easily controlled by the mole
ratio of monohalide to
dihalide. These liquid polymers have molecular weights in the
range 600-7500 and
viscosities 2.5-1400 poise at 24C.
The most useful reaction for conversion of the liquid polymers
to the high polymer state is
that of direct oxidation. This reaction results in a linking of
the two thiols to form the
polymeric disulfide with liberation of water as a by
product.
R S S R + NaSH + NaSO3 RSNa + HSR + NaS2O3
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26
R
R n
Typical reactions are:
2 RSH + PbO2 RSSR + H2O + PbO
2 RSH + ZnO RSZnSR + H2O
2 RSH + organic peroxides RSSR + H2O
It is customary to incorporate carbon black or white fillers and
plasticizers, such as dibutyl
phthalate for enhancing properties. They are almost exclusively
used in sealing, casting and
impregnation applications. Although initially employed as
binders for rocket propellants, at
present, their largest single use is in the insulating glass
industry due to their excellent
adhesion to aluminum and glass, and inherent resistance to UV
radiation and moisture
transmission. They are used as sealants in aircraft industry,
marine and construction
applications. Other uses include dental molding compound, cold
molding compound, formed
-in-place gaskets, concrete coatings and bounding, as epoxy
flexibilizer for indoor
applications and filled molding compounds.
Silicone Rubber (SI)
In spite of their high cost silicone, rubbers have established
themselves in a variety of
applications due to a combination of properties that are quite
unique with respect to organic
elastomers. These properties are, of course, dependent upon the
unusual molecular structure
of the polymer, which consists of long chains of alternating
silicon and oxygen atoms
encased by organic groups. These chains have a large molar
volume and very low
intermolecular attractive forces. These molecules are unusually
flexible and mobile and can
coil and uncoil very freely over a relatively wide temperature
range. Chemically silicones are
polysiloxanes of the general formula:
Where R, in commercially produced polymers, is methyl, phenyl,
vinyl or trifluoropropyl
group. They are produced by hydrolysis of the appropriate
dichlorosilane (R2Si Cl2) to form
cyclic tetrasiloxanes which in the presence of suitable
catalysts produce the long chain
siloxanes.
The first types available were the dimethyl siloxanes, followed
shortly by methyl phenyl
siloxanes in which the proportion of phenyl was small, imparting
the elastomer a lower
stiffening temperature than the dimethyl polymer. The newer
types of rubber contain an
olefinic group usually vinyl, to increase the reactivity of the
polymer and provide much faster
vulcanization and more elastic vulcanizates. Requirement of
vulcanizing agents such as
reactive peroxides are less than usual and may also be
reinforced with carbon black if desired.
R
Si Si
R
O O
-
27
O
Rubber polymers in which some of the methyl groups had been
replaced by groups
containing fluorine or nitrile components became available in
the 1950s. Although the
nitrile-containing polymers failed to become commercially
significant, the fluorine-
containing polymers,
with their excellent resistance to oils, fuels and solvents have
found extensive applications in
spite of their high price. The commercial materials usually
contain a small amount (about
0.2%) of methyl vinyl siloxane as a cure site monomer, whilst
the fluorosilicone component
may range from 40% to 90%, the latter figure being more
common.
Properties: The molecular weight range of the heat vulcanizable
solid polymer is 30,000-
10,00,000. The most outstanding property of silicone elastomers
is a very broad service
temperature range that far exceeds that of any other
commercially available rubber. The
silicones can be compounded to perform for extended period at
100C to 315C under static condition and at 70C to 315C under
dynamic conditions. At 205C, the silicone rubber has an estimated
useful life of 2 to 5 years, whilst most organics will fail within
a few days.
Silicone rubber performs unusually well when used as a gasket or
O-ring in sealing
applications. Over the entire temperature range of 85C to 260C,
no available elastomer can match its low compression set. The
phenyl methyl polysiloxane elastomers have
stiffening temperatures some 30-40C lower than the dimethyl
polysiloxane.
Silicone rubbers are inert chemically, have no taste or smell,
and are, with few exceptions,
physiologically acceptable to animal tissue. They are unaffected
by atmospheric exposure
and do not show ozone cracking. Many types of wires and cables
are insulated with silicone
rubber, mainly because its excellent electrical properties are
maintained at elevated
temperatures. The high permeability to gases is utilized
medically for making oxygen
permeable diaphragms. Its inertness, non-toxicity and
biocompatibility are utilized to make
medical tubings and surgical implants in human body.
Compounding: The silicone rubbers do not have very good physical
properties; in fact they
show the lowest pure gum strength of all rubbers. Therefore they
have to be reinforced in